In 2010 and 2011 we found October a LOT less humid than September even though the temps were similar. Usually you get more rain in September and very little in October. We left before the rain started last year around the 9th or so, but hadn't seen a drop of rain until our departure day and our previous year's October trip was totally rain free.
This year we had to go a month earlier and have just come back. It rained a LOT, but I didn't find it nearly as hot as last year.
Crowd levels: We have always noticed an increase in crowds from the first week of October. That said, with the longer park hours in October, I doubt I would ever do September again if October is an option. And the Food and Wine Festival is an added bonus (we missed it by a week or so this year).
The parks were dead quiet this year, with the exception of MK on non party days once the parties started on the 11th. But it's always like that when there are limited days for guests with non hopper tickets to choose their MK day so we expected that - been there, done it all before.
Given the choice, I'd go October if possible. YOu can't tell what the weather is going to do based on what it did the year before. The weather patterns seem to be more and more unstable the last few years so don't use the weather as the primary reason for your time of year to visit.